Electric ship propulsion



July 10, 1923.

A. H. MITTAG ELECTRIC SHIP PROPULSION Filed Sent. 24. 1921 Patented July 10, 1923.

'UNITED STATES PATENT ()FFICE.

ALBERT H. MI'ITAG, 0F SCHENECTADY, NEW YORK, .ASSIGNOR T0 GENERAL ELECTRIC COMPANY, A GORPQRATION OF NEW YORK.

ELECTRIC SHIP PROPULSION.

Application filed September 24, 1921.

To all whom it may concern:

Be-it known that I, ALBERT H. Mrrmo, a citizen of the United States, residing at Schenectady, county of Schenectady, State of New York, have invented certain new and useful Improvements in Electric Ship Propulsion, of which the following is a specification.

My invention relates to electric ship propulsion and particularly to systems of electric ship propulsion wherein a ships propeller is driven by an alternating current motor operating as an induction motor for maneuvering operations requiring a large torque, such as acceleration and reversal of the ship, and wherein the induction motor is converted to a synchronous motor for normal synchronous operation.

An object of my invention is to provide a convenient and efiicient means for supplying the low voltage direct current excitation re uired in a system of the class described and a further object is to provide a system which will automatically vary the excitation of the motor in accordance with the load requirements, thus increasing the maximum load that may be carried before the motor will fall out of step.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims. 7

The accompanyin drawin illustrates diagrammatically a 5 ip propu sion system embodying my invention.

Referring to the drawing the. prime mover 1, which may be, for example, an adjustable s eed turbine, is directly connected to t e synchronous generator 2, which is connected by the 111311153130 the propeller driving motor 4, the rotatable ele ment of which is directly connected to the shi s propeller 5. Suitable circuit contro ling and reversing switches 6 may be provided in the mains 3 for controlling the direction of operation of the motor 4: 1n a manner well known in the art, The motor 4-. will be provided with a phase wound secondary connected to the slip ring 7 in a Serial No. 502,925.

manner well known in the art. Suitable means may be provided for closing the circult of the phase wound secondary of the motor for induction motor operation. As illustrated in the drawing, a switch 8 is provided which when thrown to the lower position connects the slip rings 7 of the motor to a suitable startin resistance 9. Suitable means may be provided for reducing the value of the resistance 9 in order to increase the torque of the motor near synchronism. In the drawing a switch 10 is shown for short circuiting the resistance 9. When the motor has been brought practically to synchronism l disconnect the slip rings 7 from the resistance 9 and connect a source of direct current excitation to these slip rings to bring the motor into synchronism. According to my invention a rotary converter 11 is provded for furnishing the direct current excitation to the motorr The alternating current slip rings 12 of the ro tary converter are arranged to be connected in series relation with the mains 3 by means of current transformers 13. A switch 14 may be provided for short circuitin the current transformers while the rotary is ino erative. lhe direct current brushes 15 oi the rotary are arranged to be connected to the slip rings 7 when the switch 8 is thrown to its upper position. The rotary converter 11 will preferably be provided. with a separately excited shunt field winding 16 connected to a suitable source of excitation such, for example, as the constant voltage source for supp ying the excitation for the synchronous generator 2. A switch 17 will be provided for interrupting the circuit of the field winding 16 and suitable means such as a rheostat 18 will be rovided for adjusting the strength of t e field inding.

The operation of the system will be clear from a brief explanation of a method of accelerating the motor and an ex lanation of the operation of the system or constant speed and variable/load and for variable speed. To start the system shown, into operation, the turbine 1 will be started and the line switches 6 closed for the desired direction of operation. At this phase of the operation the switch 14 will be closed thus short circuiting the current transformers- 13. The switch 17 will be open thus deenergizing the rotary and the switch 8 will be in its lower position disconnecting the rotar from the slip rin 7 and connecting the ip rings to the resistance 9. Excitation will then be applied to the main generator and the motor 4 will start as an induction motor and during this phase of operation the synchronous generator may be over-excited to produce a strong induction motor tor ue in the manner well known in the art. hen the motor has accelerated sufficiently the resistance 9 may be short circuited by the switch 10 and the motor will run nearly in synchronism with the generator at which time the excitation on the generator may be reduced to normal. The rotary converter 11 will then be started as an induction motor by opening the switch 14. When the converter comes up to speed switch 17 will be closed thus synchronizing the rotary converter with the generator 2. The switch 8 is then thrown to its up r position thus removing the short circuit from the slip rings 7 and connecting the direct current brushes 15 of the rotary with the phase wound secondary of the motor 4. The motor 4 will thus be brought into exact synchronism with the generator 2. By reason of the inherent action of the combination comprising the current transformers 13 and rotary converter 11 the direct current excitation supplied to the motor, now operating as a synchronous motor, will vary in accordance with the current supplied by the generator 2 to the motor 4. This operation is desirable for the reason that increases in load on. the motor, whether due to head winds, pitching of the ship or movements of the helm require an increase in the motor field excitation to prevent the motor from falling out of step.

The-use of a rotary converter for supplying the excitation for synchronous operation of a motor of the 'type described is advantageous for the reason that the voltage required is uite low and it would be undesirable in a s ip propusion system to provide a special direct current generator for furnishing so low a voltage and the regulation of such a generator for the changes in excita tion required would be inconvenient. A rotary converter, however, may be efiiciently designed for this low voltage and when connected to the current transformers inherently provides such changes in voltage as are necessary for an approximately correct variation of the motor field current.-

Speed changes of the prope ler driving motors are customarily secured in ship propulsion systems by varying the frequency of the generator, as for example, by varying the motors will fall ofi one-half and the exciting current for the motor should be decreased approximately one-half. The converter will give approximately the required regulation of the motor field current but owin to the, change in reactance of the circuit supplied by the current transformers with the reduction of speed, the regulation may not be rigorous y' accurate. changes in excitation as ma be required, may be secured by chan ing t e field excitation of the rotary by ad ustment of the rheostat 18.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. An electric ship propulsion system comprising an adjustable speed synchronous'generator, a propeller driving motor provided with an armature having a hase winding adapted to be supplied with direct current for synchronous operation, a rotary converter having its alternating current end connected in series relation between said generator and motor and means for connecting the direct current end of said rotary converter to said phase winding for synchronous operation of said propeller motor.

2. An electric ship propulsion system comprising an adjustable speed synchronous generator, a propeller driving motor provided with a phase wound secondary, a. rota converter having its alternating current en connected in series relation between said generator and motor, a starting resistance or said motor secondary winding and switching means whereby said secondary winding may be connected either to said starting resistance for induction motor 0 eration or to the direct current end of said rotary converter for synchronous operation.

3. An electric ship propulsion system comprising an adjustable speed synchronous generator a propeller driving motor provided with a phase wound secondary, a rot converter provided with a separately excited shunt winding, current transformers.

connected to supply current to said rotary converter proportional to the current supplied from said generator to said motor and switching means whereby said secondary winding may either be closed for induction motor operation or connected to be supplied from the direct current end of said rotary converter for synchronous operation.

4. The method of operating a ship propulsion system comprising a synchronous generator, a propel or driving motor provided with a phase wound secondary, and a rota converter with its-alternating current on? connected by means of current transformers in series between said generator and motor, which consists in bringing said motor converter to supply direc t current to said wcondarg windlng for synchronous opem- Mon of me propeller driving motor. 1 In Witness whereof I have hereunto set my hand this 23rd day of September, 1921.

ALBERT H. MITTAG. 

